1. Field of the Invention
This invention relates generally to devices used to reduce or eliminate the effect of a condition known as foot-drop, and also to devices used in the rehabilitation of such a condition.
2. Description of the Prior Art
Foot-drop is a neuromuscular disability whereby certain muscles in the leg are incapable of or debilitated in lifting the foot through the swing phase of a person's walking cycle. Such a condition arises in many cases as a result of strokes which damage the electrical paths from the brain to the lower extremities leading to paralysis of the muscles in that region. Persons suffering from multiple sclerosis, traumatic injuries or other diseases involving neuromuscular damage may also experience the troubling effects of this condition.
The motions of the ankle are dorsiflexion (raising the foot) and plantar flexion (lowering the foot), and the motions of the mid-tarsal joint are inversion (turning the foot inward) and eversion (turning the foot outward). When muscles of the ankle and mid-tarsal joints are paralyzed to any extent, the rest of the body must compensate in order to overcome the effects of foot-drop. Because of the independent motion required of these joints, such attempts are rarely effective in alleviating the tendency of the foot and toes to drag as the leg is swung forward during the gait. This invention focuses on the dorsiflexion motion of the ankle, since it is the decreased ability of performing this action that is the most problematic aspect of living with a foot-drop disability.
The muscles in the leg which contribute to the dorsiflexion of the ankle are the tibialis anterior, the extensor digitorum longus, and the extensor hallucis longus. All of these muscles originate at the anterior surface of the fibula, but have separate insertion points. The tibialis anterior inserts on the inner side of the medial cuneiform bone and the base of the first metatarsal. The extensor digitorum longus inserts into the dorsal surfaces of the phalanges of the second and fifth toes, and the extensor hallucis longus inserts at the base of the distal phalanx of the big toe. The strongest muscle of the three, and hence the one most used in dorsiflexion, is the tibialis anterior. For this reason, the operation of a preferred embodiment of the invention will focus on the use of this muscle as the source of biofeedback.
A wide variety of devices have been devised to alleviate the effects of a foot-drop condition. One such apparatus consists of a rigid vertical member formed to cup the calf and heel with a V-shaped strap attached to the shoe. Another device employs a calf collar with a rigid vertical member having a bent and resiliently biased section insertable above the heel of a shoe. A more complicated apparatus use short-leg braces with torsion members coiled about a horizontal pin through the heel of the shoe. Still other devices are as simple as a calf collar with an elastic strap or ligament connected to either the big toe or a clip on the top of the shoe.
Although all of the prior art devices contribute to lifting the foot through the walking cycle, none of these are responsive to the strength remaining in the debilitated muscles. The resultant disadvantage, therefore, is that the affected muscles are not used when the device is worn, and atrophy and further degeneration of these muscles is the inevitable consequence. Likewise, in those devices which do not exercise the leg in plantar flexion, a similar effect will be seen in those muscles. This invention succeeds in overcoming these disadvantages by relying on the person's own residual muscle activity in the form of electrical biofeedback to operate the invention. This method and apparatus will thereby increase the chances of permanently alleviating foot-drop by rehabilitating the dorsiflexor muscles over time simply through using the device to assist in walking.
In order to fully understand the functioning of the invention, a brief explanation of the biomechanics of the leg and the phases of the human gait cycle is necessary. Normal walking consists of two major phases: the stance phase and the swing phase. As can be seen from FIG. 9, The stance phase occupies 60% of a single gait cycle beginning with the strike of the heel on the ground (heel-strike) and concluding with the toes leaving the ground (toe-off). Subphases of the stance are, respectively, heel-strike, foot-flat, heel-off and toe-off. The remaining 40% of the gait is between toe-off and heel-strike and is referred to as the swing phase. It is during this phase that it becomes necessary for the foot to be pulled upward so that a proper heel-strike can be made at the conclusion of the swing phase. Because foot-drop prevents such a responsive motion, this invention is provided which will overcome this effect by becoming mechanically active during the swing phase of the gait cycle. Precise timing is critical to accomplishing this task, and a logical solution to the problem is to use whatever residual activity exists in the debilitated muscles that control such motion to trigger the mechanical function of the invention. Electromyographic activity and biofeedback provide such a solution, and a brief explanation of this process follows.
Detectable muscle activity stems from chemical reactions taking place within the muscles during contraction. The muscle fibers are encased in membranes which are polarized due to these reactions, and are electrically positive on the outside with respect to the interior of the fiber. The membrane undergoes depolarization immediately prior to contraction so that this area becomes electrically negative with respect to the remainder of the muscle. The effects of this process are evidenced by a voltage existing between two electrodes placed on the skin above the subject muscle. Such a voltage is commonly referred to as electromyographic (EMG) activity or EMG voltage.
Periods of muscle activity are always followed by rest in order to minimize the expenditure of energy. Muscle activity is highest at the beginning of the stance phase and decreases through mid-stance. In late-stance, however, activity increases in preparation for the swing phase where the dorsiflexors lift the foot to clear the ground. When the heel strikes the ground at the end of the swing, considerable angular momentum is developed and a sharp muscle contraction is required to decelerate the foot. The relationship, therefore, between the level of EMG activity and the location of the foot throughout the gait cycle is predictable and is used in this invention as one of the two criteria for operating the orthosis. As explained herein, the other criterion for activation of the orthosis is based on the position of the leg during the gait cycle as detected by a simple potentiometer. Satisfaction of both of these criteria results in activation of the orthosis in an embodiment of the invention, and a rehabilitative effect should be seen over time as the user strengthens the dorsiflexor muscles used in the biofeedback process.